The present invention relates to an image forming apparatus such as a copy machine, a printer, a facsimile and the like, in which various kinds of images are formed on a sheet recording medium, and particularly, to a jam processing technique when an anomaly in transportation (a jam) of a sheet recording medium occurs.
There has been available image forming apparatuses each forming an image in a procedure in which a toner image formed on an image carrier is transferred onto a sheet recording medium on a transfer carrier by electrostatic attraction and thereafter, the toner image is fixed on the sheet recording medium in a fixing device. Of such image forming apparatuses, especially in a tandem type image forming apparatus in which plural image forming stations each including an image carrier and an image forming process means placed therearound are disposed along a direction in which a sheet recording medium is transported, a single transfer carrier is formed opposite the image forming stations and therefore, the transfer carrier is eventually long.
Therefore, in a case where a jam occurs during transportation of sheet recording media being continuously fed, which leads to an emergency shutdown of an image forming apparatus, a sheet recording medium stops between an image carrier and a transfer carrier in more of cases. Especially in a case where a jam occurs in a fixing section, the operation has to be urgently shutdown. In such a case, a possibility is high that a subsequent sheet recording medium stays stopping between an image carrier and the transfer carrier.
In such a case where a sheet recording medium stays unmoving between an image carrier and the transfer carrier, it is difficult to take out the sheet recording medium, left unmoved, by pinching it between fingers since it is firmly attracted to the transfer carrier by static electricity and since the image carrier and the transfer carrier are in contact with each other. Therefore, for example, proposals have been made on a paper transport device or a non-peelable paper processing device in which if a jam occurs, a jam processing is performed in a way such that the transfer carrier is moved in a direction opposite a moving direction in copying operation to thereby return the sheet recording medium to a position where the sheet recording medium can be taken out with ease (see, for example, JP-A Nos. S62-264144 and H07-281534).
A proposal has been made on an image forming apparatus in which in a case where a jam of a sheet recording medium occurs, only a fixing section is caused to be out of operation and the other sections are continued to operate for a predetermined time, to transport the sheet recording medium to before the fixing section and to thereby facilitate a jam processing (see, for example, JP-A No. H05-053405 paragraph [0013] and [0016]).
A proposal has been made on a image forming apparatus in which in a case where a jam occurs and a sheet recording medium is transported to before a fixing section, a transfer voltage (a transfer bias) applied to the sheet recording medium with a transfer carrier is disconnected to alleviate an attraction force between the sheet recording medium and the transfer carrier by some amount and to thereby facilitate a jam processing (see, for example, JP-A No. H11-119490 paragraph [0055] and [0066]).
However, it is difficult to visually find a sheet recording medium present between a transfer carrier and an image carrier and it is common not to install a means for detecting a sheet recording medium present on the transfer carrier in most of image forming apparatuses currently available. Hence, when a jam occurs, it is difficult for an image forming apparatus itself or for a user to determine whether or not a sheet recording medium is in a state being left firmly attracted to the transfer carrier.
Therefore, if a transfer carrier is always moved back when a jam occurs despite a high possibility of the absence of a sheet recording medium on the transfer carrier as described in JP-A Nos. S62-264144 and H07-281534, a wasteful time is forcibly consumed in some case, leading to an issue of being uneconomical. In a case where a detector detecting a sheet recording medium present on a transfer carrier is installed, problems arise that an image forming section is large in size and that increase occurs in manufacturing cost or assembling cost.
In a case of JP-A No. H05-053405, if a jam of a sheet recording medium occurs, only a fixing section is caused to be out of operation and the other sections are continued to operate for a predetermined time, to transport the sheet recording medium to before the fixing section and to thereby facilitate a jam processing. In JP-A No. H11-119490, it is disclosed that in a case where a jam occurs, application of a transfer voltage (a transfer bias) to the sheet recording medium through a transfer carrier is ceased when a sheet recording medium is transported to before a fixing section to thereby alleviate an attraction force between the sheet recording medium and the transfer carrier by some amount and to thereby facilitate a jam processing.
An attraction force between a transfer carrier and a sheet recording medium depends not only on a transfer bias but also greatly on a contact pressure with a charged image carrier; therefore, only an operation to cease application of a transfer bias, in a case, does not sufficiently reduce an attraction force of the sheet recording medium to a transfer carrier.
An attraction force of a sheet recording medium is altered greatly by an environmental condition. Therefore, such an alteration, in turn, alters a degree of difficulty in operation of peeling-off a sheet recording medium from a transfer medium in a jam processing. If an attraction force of the sheet recording medium is low, a failure in transportation occurs, while if the force is high, the sheet recording medium is difficult in being peeled off.